This invention relates to an auto-tensioner for applying tension to a timing belt, a belt for driving an auxiliary of a vehicle engine or the like while damping belt reaction force as external force, in particular, relates to measures for simplifying a damping mechanism.
Conventionally, for an auto-tensioner of such kind, there has been commonly used an auto-tensioner using a hydraulic damping mechanism as shown in the Japanese Utility Model Laying Open Gazette No.4-66448. This auto-tensioner essentially consists of: a hydraulic damping mechanism having a piston rod movable forward and backward; and a rotating member which has a tension pulley for pressing a belt and changes the operating direction of the piston rod by, for example, 90.degree..
The damping mechanism has: a cylinder body which is filled with operating oil; a piston which is inserted reciprocatably into the cylinder body and sections the inside of the cylinder body into first and second oil rooms; a piston rod passing through an end wall of the cylinder on a first oil room side and connected at an inner end thereof to the piston so as to move together with the piston; and a compression coil spring contractedly placed in the second oil room for forcing to press the piston on the first oil room side so as to extend the cylinder. A communication passage for establishing communication between the first and second oil rooms is provided in the piston and the cylinder. A check valve is provided in a part on the piston side of the communication passage.
The check valve operates to close the communication passage to restrict the movement of the piston, when the piston moves opposite to the direction that the compression coil spring forces to press by the external force in the direction of retraction of the cylinder so that the operating oil of the second oil room may flow into the first oil room. On the other hand, the check valve operates to open the communication passage to allow the movement of the piston, when the piston moves in the direction that the compression coil spring forces to press at the extension of the cylinder so that the operating oil of the first oil room may flow into the second oil room. Further, when the belt reaction force against the pressing of the tension pulley presses the piston opposite to the direction that the compression coil spring forces to press so that the external force in the direction of retraction of the cylinder is applied to a tip end of the piston rod, the check valve restricts the movement of the piston thereby damping the belt reaction force through the rotating member.
However, since the above hydraulic auto-tensioner generates the following problems, it has been desired to solve those problems.
(1) Because of the use of flow resistance of the operating oil, the above auto-tensioner requires high sealing performance thereby complicating the structure. This invites increase of the number of parts and much expense in time and effort for assembly work. Accordingly, it is difficult to reduce the cost. PA1 (2) In the case of application of excessive belt reaction force, the pressure of the operating oil is increased in proportion to the reaction force, so that the damping mechanism may be damaged due to high pressure. PA1 (3) Since damping characteristics are determined by passage resistance of communication passage, check valve and the like, specified damping characteristics are previously established in the damping mechanism. Accordingly, it is difficult to change and adjust the established damping characteristics. To change the damping characteristics, the whole damping mechanism must be exchanged. PA1 (4) Viscosity characteristics of operating oil which have an effect on damping characteristics are readily affected by change of temperature. For example, flow resistance of operating oil is increased at a low temperature as compared with at an ordinary temperature, so that damping force excesses its set value. PA1 (5) Since the damping mechanism operates in the direction of the length, in the case where the tension pulley is directly mounted at the tip end of the piston rod to press the belt, the base end of the damping mechanism protrudes sideward from the engine. Therefore, the damping mechanism must be placed along a running direction of the belt, thereby requiring another rotating member for changing the operating direction of the piston rod. This prevents compaction and weight reduction of the auto-tensioner. PA1 (6) For the same reason as in the above point (5), two members of the damping mechanism and the member for changing the operating direction of the piston rod must be mounted on the engine. This invites much expense in time and effort for assembly work.
This invention has been made in view of the foregoing problems. It is a main object of this invention, by using pulley members and a friction member such as a belt wound therearound, to achieve easy assembly at a low parts count to reduce the cost, to provide a fail-safe function with respect to excessive external force, to achieve easy tuning for setting damping characteristics and reduction in temperature-dependent performance of damping characteristics, and to simplify assembly work to the engine while reducing in size.